In the field of semiconductor wafer manufacturing, it is necessary for a wafer to undergo a so-called “wet clean” processing stage prior to patterning of gates. This processing stage is an important stage in the manufacture of semiconductor wafers, because cleanliness of the surface of the wafer at this early stage of manufacture impacts upon defectivity and hence yields of semiconductor devices ultimately made.
Historically, the wet clean stage comprises two cleaning step. Using a first wet bench, a first step requires wafers to undergo a so-called “RCA clean” using a Standard Clean 1 (SC-1) cleaning solution. The SC-1 cleaning solution has an alkaline pH and comprises ammonia, hydrogen peroxide and water. After the RCA clean, a second step of the wet clean stage employs a second wet bench to immerse the wafers in a Standard Clean 2 (SC-2) cleaning solution comprising hydrochloric acid, hydrogen peroxide and water, the SC-2 cleaning solution having an acid pH.
The SC-1 cleaning solution is highly efficient at removing particles and organic residuals, due to a high Zeta potential in the alkaline range of dielectric surfaces commonly forming part of a surface of a semiconductor wafer. However, the SC-1 cleaning solution is ineffective at removing metallic contamination, such as residues. The second, acidic, clean part of the wet clean stage using the SC-2 cleaning solution is therefore necessary, and use of the wet benches moreover allows batch cleaning of semiconductor wafers.
Wafer sizes are now changing and a migration to 300 mm diameter wafers is taking place. A necessary result of the migration to the 300 mm wafer size is the replacement of the above-described wet benches with single wafer processing tools to avoid cross contamination between wafers being cleaned simultaneously in a given wet bench. This change to single wafer processing tools to wet clean the wafers poses a new challenge to users of the wet clean step, since in order to maintain the throughput of wafers in the wet clean step, each individual wafer has to be cleaned in about 30 seconds as opposed to the 10 minutes taken previously when the wafers were batch cleaned. In order to facilitate the reduced process time per wafer required, it is desirable to combine the functions of the alkaline and the acidic cleaning steps.
U.S. Pat. Nos. 6,143,706, 6,228,179, 6,228,823 and 6,498,132 disclose possible cleaning solutions to reduce the cleaning time of wafers in the single wafer processing tools. The cleaning solutions disclosed operate in the alkaline range of pH to remove particulate contaminants with high efficiency. Efficiency of removal of metallic residues is addressed by employing a variety of complexing agents in the cleaning solution, the complexing agents typically containing ethylene-diamine and derivatives thereof.
Whilst the above-mentioned cleaning solutions show promise for several cleaning applications, in cleaning stages where efficiency of metal removal is critical, the above-mentioned cleaning solutions do not always provide sufficient cleaning efficiency for some applications to avoid compromising the (Charge-to-Breakdown) reliability of devices being formed.
It is, of course, known that removal of metallic species in alkaline media is difficult due to low solubility of the metallic species in the alkaline media and the high affinity of the metallic species to surface silanol groups at high pH. Removal of the metallic species is therefore driven by completing agents, though the complexing agents have to be highly efficient in order to remove the metallic species from the surface of the wafer in such a short processing time. In order to improve the efficiency of removal of the metallic species, U.S. Pat. No. 6,498,132 therefore proposes the use of multiple complexing agents.
However, even with the use of multiple complexing agents, removal of metallic species from the surfaces of wafers is still inefficient. Prior to the wet clean stage, the surface of a semiconductor wafer is hydrogen terminated, resulting in the surface of the wafer being hydrophobic. When cleaned in an alkali media, silicon on a surface of the semiconductor wafer undergoes oxidation resulting in the surface of the semiconductor wafer acquiring hydrophylic wetting properties. Consequently, the accessibility of the complexing agents to surface sites of the wafers is hindered by the inhomogeneous wet-ability of the surface of the semiconductor wafer during the oxidation of the silicon.